1. Field of the Invention
This invention relates to optical phase filters, and more particularly to optical phase filters using liquid crystals capable of varying the imaging performance of cameras and other optical instruments.
2. Description of the Prior Art
An optical phase filter 1 as shown in FIG. 1 has been employed for lowering the sharpness of the image formed by lens or the like. In FIG. 1, a glass substrate 2 is provided with a great number of transparent round convex phase portions 3 of silicon oxide or the like. This optical phase filter is generally called a "soft focus filter" and when used as an attachment to the front of the objective lens of a camera, can lower the image sharpness so that soft portrayal photographs, which are said to be psychologically pleasant, are obtained.
Analyses have already been made of how the optical phase filter affects the image sharpness, or MTF, at the pupil of an imaging optical system. Assuming that the phase portions 3 of FIG. 1 have a value .delta. and their distribution is at random, the curve shown in FIG. 2 is obtained for MTF versus Space Frequency. Specifically, a portion A-B, in which the mutual relation of randomness (MTF) decreases, joins with another portion B-C, in which the randomness has a constant mutual relation. However, the mutual relation of the aperture of the pupil only works at a turning point B. This turning point B can be defined in the coordinates of FIG. 2 by EQU Fb.apprxeq.a/(f.multidot..lambda.) (1) EQU Mb=.vertline.Ao+A.delta..multidot.exp[-ik.delta.].vertline..sup.2 .thrfore.Ao+A=1 (2) EQU k.multidot..delta.=2.pi./.lambda..multidot.(n-1)d (3)
where a is the average of the diameters of the phase portions 3; f is the focal length of the image forming lens; and .lambda. is the wavelength of light. Also A.delta. is the ratio of the total sum of the areas of the phase portions 3 to the entire area of the filter; and Ao is the ratio of the phaseless area (zero phase) to the entire area. The phase .delta. represents the difference between the lengths of the optical path through the phase portion 3 and the phaseless portion. In these equations n is the refractive index of the phase portion, and d is the geometric thickness of the phase portion. The point B can be fully determined using equations 1 to 3 and a certain combination of values of the parameters .lambda., Ao, A.delta., a, f, and .lambda..
When Ao=A.delta.=0.5, EQU Mb=cos.sup.2 (k.delta./2) (4)
For .delta.=5.lambda.g=0.5.times.550 [nm], the MTF value Mb at the point B varies with wavelength as shown in FIG. 3. Where .delta.=0.5.lambda.g, the phase filter 1 can lower the MTF throughout the entire visible light region.
However, the camera lens does not always obtain a soft portrayal. Occasionally, sharp representations result. Therefore, conventional practice has been to attach or detach the optical phase filter 1 to or from the lens as needed. Also, the optical phase filter has been replaced by another one to change the degree of softness of the portrayal. In some photographic situations, a photographer may desire to change rapidly between soft and sharp portrayal shootings, or to change rapidly between the different degrees of softness in successive shootings. In such cases, the use of conventional optical phase filters did not meet that need, because quick and easy attaching or interchanging of the filters was very difficult to perform.
It has been proposed to use liquid crystals for constructing an optical phase filter so that quick and easy change between the soft and sharp representations is possible. For example, in Japanese Laid-Open Patent Application No. SHO-50-84236 (published July 9, 1975), transparent electrodes on either side of the liquid crystal layer are provided with regular array of holes to make the liquid crystal carry out the above-described function. However, this prior known filter has the drawback that the soft focus effect is produced only in a particular direction.
An object of the present invention is to eliminate the above-described problems, and to provide an optical phase filter using a liquid crystal which, when once attached, is able to change over between a soft portrayal over the entire area of the image and a sharp portrayal without having to be detached.
To accomplish this object, according to the present invention, the filter is constructed with a liquid crystal layer of which the structure changes between the homogeneous configuration and the homeotropic configuration depending upon the presence or absence of application of an electric field. Two transparent electrodes apply the electric field to the liquid crystal layer. At least one of the two transparent electrodes is provided with a great number of holes distributed at random over the entire area thereof so that the MTF value is made to vary by the double-refracting property of the liquid crystal.